- Email: [email protected]
Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method
Accepted Manuscript Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method Yongyu Li, Haijie Sun, Ning Wang, Wenxue Fang, Zhongjun Li PII:
S1293-2558(14)00190-3
DOI:
10.1016/j.solidstatesciences.2014.08.003
Reference:
SSSCIE 4985
To appear in:
Solid State Sciences
Received Date: 1 April 2014 Revised Date:
31 July 2014
Accepted Date: 7 August 2014
Please cite this article as: Y. Li, H. Sun, N. Wang, W. Fang, Z. Li, Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method, Solid State Sciences (2014), doi: 10.1016/j.solidstatesciences.2014.08.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method 1
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Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li
ACCEPTED MANUSCRIPT
Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li1* College of Chemistry and Molecular Engineering, Zhengzhou University,
zhengzhou 450001, RP China 2
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1
Institute of Environmental & Catalytic Engineering, Department of Chemistry,
Zhengzhou Normal University, zhengzhou 450044, RP China
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To whom correspondence should be addressed.
Phone/Fax: +86-0371-67783123. E-mail: [email protected]
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Abstract
PbTiO3 photocatalyst was synthesized successfully by facile hydrothermal method. The effects of the hydrothermal reaction temperatures and the pH values of the systems on the photocatalytic activities of PbTiO3 were investigated in detail. The photocatalytic activities of samples were evaluated by the degradation of methyl
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orange (MO) aqueous solution under simulated solar irradiation. The as-obtained PbTiO3 sample exhibits anisotropical growth along the (0 0 1) plane, and its photocatalytic activity is about 3 times higher than that of PbTiO3 prepared by precipitation method. Moreover, the as-prepared PbTiO3 has high stability during
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photocatalytic oxidation process, and does not cause secondary pollution. Keywords: PbTiO3; photocatalysis; hydrothermal
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1. Introduction
PbTiO3, a perovskite oxide, is an important material for its piezoelectricity,
ferroelectricity, and colossal magnetoresistivity, which makes it highly attractive for fundamental
research
and
various
technical
applications
[1-4].
Recently,
perovskite-type oxide materials based on transition metals with d(0) and d(10) electron configuration such as Nb(V), Ti(IV) and In(
) were reported as efficient
photocatalysts for overall water splitting with high quantum yields [5]. Pb2+-containing transition-metal oxides were found to be able to absorb visible light due to their smaller bandgap sizes [6]. PbTiO3, a d(0) and d(10) metal oxide, can be 1
ACCEPTED MANUSCRIPT used as an efficient visible-light photocatalyst for overall water splitting [7]. To date, PbTiO3 was prepared successfully by high reaction temperature methods such as solid-state synthesis [8-10] and molten-salt flux techniques [7], which are usually characterized by high cost, large particle sizes and low surface areas of the
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products. It is believed that nanoscale materials perform higher photocatalytic efficiency than their bulk counterparts due to the larger surface area and the faster arrival to the reaction sites of the photogenerated electrons and holes. So developing effective synthetic routes for PbTiO3 with nano-scale at low temperature is
SC
significative to improve its photocatalytic performance. Compared with other methods, hydrothermal synthesis is advantageous because the particle size and morphology can
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be effective modulated, which also enables the growth of PbTiO3 with perovskite structure far below the transition temperature [11]. Therefore, a facile hydrothermal method carried out at a relatively low temperature was selected to demonstrate the possibility of preparing PbTiO3 with nanostructures and to improve its photocatalytic activity. The photocatalytic performance of products were evaluated by decomposing
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methyl orange (MO) under simulated solar irradiation at room temperature. The effects of hydrothermal temperatures and the suspension pH values on the photocatalytic activities of products were investigated.
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2. Experimental
2.1. Sample preparation
Tetrabutyl titanate ((C4H9O)4Ti) and lead nitrate Pb(NO3)2 were used as the
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starting materials. All the reagents were of AR grade and were used without further purification. In a typical synthesis, Pb(NO3)2 was dissolved in deionized water to form solution (1 mol L−1), the pH value of the solution was adjusted by 3 mol L−1 NaOH solution under vigorous stirring. The mixture was then stirred for 0.5 h in an ice bath. The ethanol solution of (C4H9O)4Ti (1 mol L−1) was added dropwise. After being further stirred for 1 h, the resulting precursor precipitate was transferred into a Teflon-lined autoclave (100 mL) and heated at different temperature for 24 h under autogenously pressure, and then air cooled to room temperature. The resulting precipitates were filtrated, washed with ethanol and deionized water thoroughly and 2
ACCEPTED MANUSCRIPT dried at 80
in air. To investigate the effects, the pH values were set at 12.95, 13.10,
13.25 and 13.40, and the hydrothermal temperatures were set at 80 200
and 220
, 120
, 160
,
, respectively.
For comparison, PbTiO3 was also synthesized by precipitation method as
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following: Pb(NO3)2 and Ti(OC4H9)4 as the raw materials, ammonia was used as the precipitating agent to adjust the pH value to be 10.0. After stirred for 1 h, the precursor precipitates were filtrated, washed and dried at 80 500
for 2 h to obtain products.
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2.2. Characterization
, and then calcinated at
The X-ray diffraction (XRD) patterns of the samples were measured on an
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X’Pert PRO X-ray powder diffractometer with Cu-Kα radiation (λ = 1.5418 A˚). The microstructure of the sample was investigated by transmission electron microscopy (TEM, JEOL JEM-2100). UV–vis diffuse reflectance spectrum (DRS) was obtained using a UV–vis spectrometer (Shimadzu U-3010) by using BaSO4 as a reference. Nitrogen adsorption–desorption isotherms were collected on a NOVA 1000e surface
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area and porosity analyzer (Quantachrome, USA) at 77K after the samples had been degassed in the flow of N2 at 150 using desorption data.
for 1.5 h. The BET surface area was estimated
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2.3. Photocatalytic activity test
The photocatalytic performance of the as-prepared samples were characterized by decomposing methyl orange (MO) under simulated solar irradiation at room
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temperature. A 500 W Xe illuminator was used as an internal light source. The photodegradation experiments were carried out with the samples (500 mg) suspended in MO aqueous solution (500 mL, 10 mg L−1) with constant stirring. Prior to the irradiation, the suspensions were magnetically stirred in the dark for 1 h to establish the adsorption/desorption equilibrium. At the given time intervals, about 5mL of the suspension was taken for analysis after centrifugation. The concentration of MO was detected by measuring the absorption intensity at a wavelength of 464 nm. The absorption intensity was converted to the MO concentration referring to a standard curve which showed a linear relationship between the concentration and the 3
ACCEPTED MANUSCRIPT absorption intensity. 3. Results and discussion 3.1. Characterization of PbTiO3 Fig.1a presents the XRD patterns of PbTiO3 samples at the pH of 13.25 with
80
and 120
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different hydrothermal temperatures. As shown in the figure, the products obtained at are amorphous, and diffraction peaks of PbTiO3 could not be
detected. When the temperature was increased to 160
, a series of sharp and clear
diffraction peaks appear, which could be assigned to tetragonal phase PbTiO3 (JCPDS
was further increased to 220
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01-78-0298). No other impurities could be detected. As the hydrothermal temperature , the diffraction peaks intensity of sample was
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increased gradually, indicating that the crystallinities of samples improve. According to the Scherrer formula, the crystallite sizes of PbTiO3 samples obtained at 160 200
and 220
,
, calculated based on the data of (101) plane, are 53.2 nm, 67.6
nm and 78.7 nm, respectively.
Fig.1b shows the XRD patterns of PbTiO3 obtained at hydrothermal temperature
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200 °C with different pH values. It can be seen that the crystallinity of products is low at the pH value of 12.95. When the pH values are greater than 13, the diffraction peaks match well with tetragonal phase PbTiO3 (JCPDS 01-78-0298), and no peaks of
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any other phases or impurities were detected. The (001) peak intensity of samples obtained at the pH of 13.25 was increased suddenly, even stronger than (101) peak intensity, which may be due to anisotropic growth along the (001) facets [12].
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According to the Scherrer formula, the sample crystallite sizes calculated based on (101) plane are 51.3 nm, 67.6 nm and 67.7 nm when the pH value are 13.10, 13.25 and 13.40, respectively. It is obvious that the crystallite sizes of samples were increased with increasing pH values. Fig. 2 shows the TEM image and high-resolution TEM (HRTEM) image of the PbTiO3 prepared at the pH of 13.25 and the hydrothermal temperature of 200 °C. The as-prepared sample displays elliptical in shape, and its particle sizes could be estimated to be about 250 – 400 nm, which is much larger than its crystallite size (67.6 nm) obtained from the XRD data. The results indicate that there are multiple 4
ACCEPTED MANUSCRIPT domain in the particles of the sample and the PbTiO3 particles observed by TEM are mainly polycrystalline [13]. To illustrate the anisotropic growth along (001) plane of the as-prepared PbTiO3, the HRTEM image and the standard XRD pattern of PbTiO3 (JCPDS 01-78-0298)
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from powder diffraction file database are provided (Fig. 2b and Fig. 2c). In the standard pattern (Fig. 2c), the intensity of the (0 0 1) peak is much weaker than that of the (1 0 1) peak, which could be expressed as I (1 1 0) / I (1 0 2) = 0.25. In our case, the intensity of the (0 0 1) peak is even beyond that of (1 0 2) peak, the value of I (0 0
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1) / I (1 0 1) was increased to 1.59. The related HRTEM image of the as-prepared sample (Fig. 2b) displays the clearly resolved crystalline domain, an uniform
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interplanar spacing of 0.415 nm correspond well to the (001) plane of PbTiO3. Furthermore, the angle labeled in the corresponding fast-Fourier transform (FFT) pattern (inset in Fig. 2b) is 46.8◦, which is identical to the theoretical value obtains for the angle between the (0 0 1) and (1 0 1) planes. The results prove that the PbTiO3 sample anisotropically grew along the (0 0 1) plane.
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UV-Vis diffuse reflectance spectrum of the as-prepared PbTiO3 samples was analyzed to investigate the optical absorption performance. As shown in Fig. 3, the absorption edge of the PbTiO3 samples synthesized at the hydrothermal temperature
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of 200 °C and the pH of 13.25 extends to the visible light region, ranging from 200 nm to 450 nm. The steep shape of the spectrum indicates that the visible light absorption is ascribed not to the transition from the impurity level but to the band-gap
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transition [14]. Its band gap could be derived using the equation αhν = A(hν - Eg)n/2, where α, A, hν and Eg signify the absorption coefficient, proportionality constant, photon energy and band gap energy, respectively [15]. Considering PbTiO3 to be an indirect semiconductor (n = 4 is adopted), the plot of (αhν)1/2 versus hν affords the bandgap of PbTiO3 as shown in the inset of Fig. 3, and then evaluate the band gap Eg by extrapolating the tangent line to the hν axis intercept. The estimated band gap energy of the resulting samples is about 2.75 eV, which is consistent with previous reports [6,7]. The BET surface areas play a great role on the photocatalytical property of 5
ACCEPTED MANUSCRIPT samples. The specific surface areas of samples were investigated by using nitrogen adsorption and desorption isotherms. The surface areas of PbTiO3 prepared at the hydrothermal temperature of 200 °C are 3.33, 2.56 and 2.52 m2 g-1 at the pH values of 13.10, 13.25 and 13.40, respectively. The BET surface areas were decreased gradually
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with increasing pH value. However, the specific surface areas of samples prepared at pH values of 13.25 and 13.40 have little difference, which is consistent with the change of their crystallite sizes. The effects of the hydrothermal temperatures were also investigated, the surface area of PbTiO3 prepared at pH value of 13.25 are 2.06,
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2.56 and 1.74 m2 g-1 at 160, 200 and 220 °C, respectively. The specific surface areas of samples were firstly increased and then decreased. When the temperature was to 220
, the specific surface area were reduced, indicating
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increased from 200
that the crystalline degree was increased with increasing temperature, and accordingly specific surface area was decreased. 3.2. Photocatalytic performance
Fig. 4 shows the photocatalytic degradation of MO over PbTiO3 samples
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prepared at different conditions. It can be seen from Fig. 4a that the photolysis of methyl orange was very slow, and only about 3% was degraded after 180 min of illumination. The degradation of MO with as-prepared PbTiO3 samples in the dark
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condition gives similar results to the photolysis test, indicating that the adsorption of MO on the PbTiO3 samples was limited after the adsorption–desorption equilibrium was reached. However, about 91% of MO was decolored after 180 min of
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illumination over the PbTiO3 samples, indicating that the product exhibits relatively high photocatalytic activity. Although there might be photolysis of MO, the results of above comparison signify that MO degradation in the present study was through a photocatalytic process.
Fig. 4a presents the photocatalytic degradation of MO over PbTiO3 samples prepared at the pH of 13.25 with different temperatures. It is obvious that the hydrothermal temperatures had a great impact on photocatalytic properties of as-prepared samples. As the hydrothermal temperature was increased from 120 6
to
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, the photocatalytic activities of products were increased gradually. It could be
ascribed to the increase of the photocatalyst crystallinity degree with increasing temperature, which resulted in the reduction of the electron - hole recombination centers. However, when the hydrothermal temperature was increased to 220
, the
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photocatalytic activity of PbTiO3 was reduced inversely. It is due to the increase of particle sizes with increasing temperature, which led to the decrease of specific surface area and the active sites, so the photocatalytic activity of product was decreased.
prepared at 200
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Fig. 4b shows the photocatalytic degradation of MO over PbTiO3 samples with different pH values. When the pH value was increased from
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12.95 to 13.25, the photocatalytic activities of samples were increased, which can be attributed to the improvement of the catalyst crystallinity according to the XRD results. As the pH was further increased to 13.40, the photocatalytic activities of products were decreased, which may be related to the decrease of the (001) peak intensity, and it needs to be further studied. Generally, the optimal condition for the
value of 13.25.
and the reaction pH
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preparation of PbTiO3 is the hydrothermal temperature of 200
For comparison, the photocatalytic activity of PbTiO3 prepared by precipitation
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method was also tested. After 180 min of irradiation, the degradation efficiency of MO was only 31%, which is much less than that of PbTiO3 prepared by hydrothermal method under the same conditions.
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The stability and reusability of catalyst is an important indicator for its practical
application. To investigate its stability, the photodegradation cycle experiments of methyl orange over PbTiO3 samples were conducted, and the experimental results are shown in Fig. 5. The experimental conditions are accordance with the photodegradation experiment. It shows that the degradation efficiency of methyl orange over PbTiO3 is still up 73% after five photocatalytic cycles. The photocatalytic activity was not reduced considerably, indicating that the PbTiO3 sample has a high stability during the photodegradation process of methyl orange. It is very important in the application of the photocatalyst [16]. However, the photocatalytic activity of 7
ACCEPTED MANUSCRIPT PbTiO3 was decreased to a certain degree through the recycling. This may be ascribed to the presence of residual intermediates on the catalyst surface which reduced the surface activity centers, and then made the photocatalytic activity of the catalyst reduced [17]. Additionally, the XRD patterns of PbTiO3 samples after the cycle
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experiments was analyzed and shown in Fig. 6. It reveals that the phase and structure remained intact, implying that the as-prepared PbTiO3 has a high stability during photocatalytic oxidation of model pollutant.
The research of PbTiO3 as photocatalyst on the degradation of organic matter is
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scarce, and one of the reasons is that it may cause secondary pollution. So the dissolubility of PbTiO3 in the photocatalytic process was investigated by inductively
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coupled plasma atomic emission spectrometry (ICP-AES). The experimental process is similar to the photocatalytic process. The light illumination time was set to 15 h, and a sample was taken to analysis the amount of Pb2+ every 3h. After 1h dark adsorption, the concentration of Pb2+ dissolved in solution was reached to 0.139 mg·L−1, and the concentration of Pb2+ was increased to 0.212 mg·L−1 after 3 h of
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irradiation. But the concentration of Pb2+ was not increased much after 12 h of irradiation, the final cumulative concentration of Pb2+ was 0.226 mg·L−1. The total lead concentration dissolved in solution after 15 h of irradiation is lower than the limit
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of 0.5 mg·L−1, which meets the National Emission Standards of China (GB 25466-2010). Therefore, PbTiO3 does not cause secondary pollution when it is
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applied to wastewater treatment as a photocatalyst. 4. Conclusions
In summary, PbTiO3 photocatalyst was synthesized successfully by a facial
hydrothermal treatment. The hydrothermal reaction temperatures and the pH values of systems have great influence on the photocatalytic activities of PbTiO3, which were evaluated by the degradation of MO under simulated solar irradiation. According to the photodegradation efficiencies, the optimal condition is the hydrothermal reaction temperature of 200
and the pH value of 13.25. The PbTiO3 prepared at the
condition could degrade about 91% of MO after 180 min of illumination. In addition, the sample is highly stable during photocatalytic oxidation process of model pollutant, 8
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Acknowledgements This work was supported by National Natural Science Foundation of China (NO.
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21001096 and U1304204) and Foundation of He’nan Educational Committee
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(NO.13A150348).
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ACCEPTED MANUSCRIPT References [1] A. Yourdkhani, G. Caruntu, J. Phys. Chem. C 115 (2011) 14797-14805. [2] M. Yashima, K. Omoto, J. Chen, H. Kato, X. Xing, Chem. Mater. 23 (2011) 3135-3137.
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[3] P. Hu, J. Chen, J. Deng, X. Xing, J. Am. Chem. Soc. 132 (2010) 1925-1928. [4] M. Park, S. Hong, J. Kim, Y. Kim, S. Buhlmann, Y.K. Kim, K. No, Appl. Phys. Lett. 94 (2009) 092901-092903.
[5] H.G. Kim, D.W. Hwang, J.S. Lee, J. Am. Chem. Soc. 126 (2004) 8912-8913.
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[6] H.G. Kim, O.S. Becker, J.S. Jang, S.M. Ji, P.H. Borse, J.S. Lee, J. Solid State Chem. 179 (2006) 1214–1218.
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[7] D. Arney, T. Watkins, P.A. Maggard, J. Am. Ceram. Soc. 94 (2011) 1483-1489. [8] V. Stankus, J. Dudonis, Mater. Sci. Eng. B 109 (2004) 178–182. [9] S. Yu, H. Huang, L. Zhou, Chem. Mater. 19 (2007) 2718-2720. [10] R. Velchuri, B. V. Kumar, V. R. Devi, G. Prasad, M. Vithal, Ceram. Int. 36 (2010) 1485-1489.
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[11] J. Moon, T. Li, C.A. Randall, J.H. Adair, J. Mater. Res. 12 (1997) 189-197. [12] Y. Li, J. Wang, H. Yao, L. Dang, Z. Li, J. Mol. Catal. A: Chem. 334 (2011) 116-122.
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[13] J.-L. Liu, L.-J. Zhu, Y. Pei, J.-H. Zhuang, H. Li, H.-X. Li, M.-H. Qiao, K.-N. Fan, Appl. Catal. A: Gen. 353 (2009) 282-287. [14] C. Zhang, Y. Zhu, Chem. Mater. 17 (2005) 3537-3545.
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[15] H. He, J. Yin, Y. Li, Y. Zhang, H. Qiu, J. Xu, T. Xu, C. Wang, Appl. Catal. B: Environ. 156–157 (2014) 35-43. [16] M. Shang, W. Wang, S. Sun, L. Zhou, L. Zhang, J. Phys. Chem. C 112 (2008) 10407-10411.
[17] L. Jing, B. Xin, F. Yuan, B. Wang, K. Shi, W. Cai, H. Fu, Appl. Catal. A: Gen. 275 (2004) 49-54.
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ACCEPTED MANUSCRIPT Figure captions: Fig. 1. XRD patterns of the PbTiO3 products prepared at different temperatures (a) and different pH values (b). Fig. 2. TEM image (a) and high-resolution TEM image (b) of the PbTiO3 prepared at
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pH = 13.25 and hydrothermal temperature 200 °C, the inset of (b) is the corresponding fast-Fourier transform (FFT) pattern. (c) XRD pattern of the PbTiO3 and standard XRD pattern of PbTiO3 (JCPDS no. 01-078-0298).
Fig. 3. The diffuse reflection spectrum of the PbTiO3 prepared at pH = 13.25 and
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hydrothermal temperature 200 °C.
Fig. 4. Photocatalytic degradation of MO over PbTiO3 samples prepared at pH =
different pH values (b).
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13.25 with different temperatures (a) and PbTiO3 samples prepared at 200
with
Fig. 5. Cycle degradation experiments of methyl orange over PbTiO3 samples.
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Fig. 6. XRD patterns of PbTiO3 samples after the photocatalytic cycle experiments.
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ACCEPTED MANUSCRIPT Research highlights:
Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method
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Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li1*
◆PbTiO3 photocatalyst was synthesized by hydrothermal method.
PbTiO3 prepared by precipitation method.
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◆The photocatalytic activity of PbTiO3 is about 3 times higher than that of the
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◆The effects of the temperatures and the pH values of systems on the photocatalytic
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activities of PbTiO3 were investigated.
S1293-2558(14)00190-3
DOI:
10.1016/j.solidstatesciences.2014.08.003
Reference:
SSSCIE 4985
To appear in:
Solid State Sciences
Received Date: 1 April 2014 Revised Date:
31 July 2014
Accepted Date: 7 August 2014
Please cite this article as: Y. Li, H. Sun, N. Wang, W. Fang, Z. Li, Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method, Solid State Sciences (2014), doi: 10.1016/j.solidstatesciences.2014.08.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method 1
AC C
EP
TE D
M AN U
SC
RI PT
Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li
ACCEPTED MANUSCRIPT
Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li1* College of Chemistry and Molecular Engineering, Zhengzhou University,
zhengzhou 450001, RP China 2
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1
Institute of Environmental & Catalytic Engineering, Department of Chemistry,
Zhengzhou Normal University, zhengzhou 450044, RP China
SC
To whom correspondence should be addressed.
Phone/Fax: +86-0371-67783123. E-mail: [email protected]
M AN U
Abstract
PbTiO3 photocatalyst was synthesized successfully by facile hydrothermal method. The effects of the hydrothermal reaction temperatures and the pH values of the systems on the photocatalytic activities of PbTiO3 were investigated in detail. The photocatalytic activities of samples were evaluated by the degradation of methyl
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orange (MO) aqueous solution under simulated solar irradiation. The as-obtained PbTiO3 sample exhibits anisotropical growth along the (0 0 1) plane, and its photocatalytic activity is about 3 times higher than that of PbTiO3 prepared by precipitation method. Moreover, the as-prepared PbTiO3 has high stability during
EP
photocatalytic oxidation process, and does not cause secondary pollution. Keywords: PbTiO3; photocatalysis; hydrothermal
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1. Introduction
PbTiO3, a perovskite oxide, is an important material for its piezoelectricity,
ferroelectricity, and colossal magnetoresistivity, which makes it highly attractive for fundamental
research
and
various
technical
applications
[1-4].
Recently,
perovskite-type oxide materials based on transition metals with d(0) and d(10) electron configuration such as Nb(V), Ti(IV) and In(
) were reported as efficient
photocatalysts for overall water splitting with high quantum yields [5]. Pb2+-containing transition-metal oxides were found to be able to absorb visible light due to their smaller bandgap sizes [6]. PbTiO3, a d(0) and d(10) metal oxide, can be 1
ACCEPTED MANUSCRIPT used as an efficient visible-light photocatalyst for overall water splitting [7]. To date, PbTiO3 was prepared successfully by high reaction temperature methods such as solid-state synthesis [8-10] and molten-salt flux techniques [7], which are usually characterized by high cost, large particle sizes and low surface areas of the
RI PT
products. It is believed that nanoscale materials perform higher photocatalytic efficiency than their bulk counterparts due to the larger surface area and the faster arrival to the reaction sites of the photogenerated electrons and holes. So developing effective synthetic routes for PbTiO3 with nano-scale at low temperature is
SC
significative to improve its photocatalytic performance. Compared with other methods, hydrothermal synthesis is advantageous because the particle size and morphology can
M AN U
be effective modulated, which also enables the growth of PbTiO3 with perovskite structure far below the transition temperature [11]. Therefore, a facile hydrothermal method carried out at a relatively low temperature was selected to demonstrate the possibility of preparing PbTiO3 with nanostructures and to improve its photocatalytic activity. The photocatalytic performance of products were evaluated by decomposing
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methyl orange (MO) under simulated solar irradiation at room temperature. The effects of hydrothermal temperatures and the suspension pH values on the photocatalytic activities of products were investigated.
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2. Experimental
2.1. Sample preparation
Tetrabutyl titanate ((C4H9O)4Ti) and lead nitrate Pb(NO3)2 were used as the
AC C
starting materials. All the reagents were of AR grade and were used without further purification. In a typical synthesis, Pb(NO3)2 was dissolved in deionized water to form solution (1 mol L−1), the pH value of the solution was adjusted by 3 mol L−1 NaOH solution under vigorous stirring. The mixture was then stirred for 0.5 h in an ice bath. The ethanol solution of (C4H9O)4Ti (1 mol L−1) was added dropwise. After being further stirred for 1 h, the resulting precursor precipitate was transferred into a Teflon-lined autoclave (100 mL) and heated at different temperature for 24 h under autogenously pressure, and then air cooled to room temperature. The resulting precipitates were filtrated, washed with ethanol and deionized water thoroughly and 2
ACCEPTED MANUSCRIPT dried at 80
in air. To investigate the effects, the pH values were set at 12.95, 13.10,
13.25 and 13.40, and the hydrothermal temperatures were set at 80 200
and 220
, 120
, 160
,
, respectively.
For comparison, PbTiO3 was also synthesized by precipitation method as
RI PT
following: Pb(NO3)2 and Ti(OC4H9)4 as the raw materials, ammonia was used as the precipitating agent to adjust the pH value to be 10.0. After stirred for 1 h, the precursor precipitates were filtrated, washed and dried at 80 500
for 2 h to obtain products.
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2.2. Characterization
, and then calcinated at
The X-ray diffraction (XRD) patterns of the samples were measured on an
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X’Pert PRO X-ray powder diffractometer with Cu-Kα radiation (λ = 1.5418 A˚). The microstructure of the sample was investigated by transmission electron microscopy (TEM, JEOL JEM-2100). UV–vis diffuse reflectance spectrum (DRS) was obtained using a UV–vis spectrometer (Shimadzu U-3010) by using BaSO4 as a reference. Nitrogen adsorption–desorption isotherms were collected on a NOVA 1000e surface
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area and porosity analyzer (Quantachrome, USA) at 77K after the samples had been degassed in the flow of N2 at 150 using desorption data.
for 1.5 h. The BET surface area was estimated
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2.3. Photocatalytic activity test
The photocatalytic performance of the as-prepared samples were characterized by decomposing methyl orange (MO) under simulated solar irradiation at room
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temperature. A 500 W Xe illuminator was used as an internal light source. The photodegradation experiments were carried out with the samples (500 mg) suspended in MO aqueous solution (500 mL, 10 mg L−1) with constant stirring. Prior to the irradiation, the suspensions were magnetically stirred in the dark for 1 h to establish the adsorption/desorption equilibrium. At the given time intervals, about 5mL of the suspension was taken for analysis after centrifugation. The concentration of MO was detected by measuring the absorption intensity at a wavelength of 464 nm. The absorption intensity was converted to the MO concentration referring to a standard curve which showed a linear relationship between the concentration and the 3
ACCEPTED MANUSCRIPT absorption intensity. 3. Results and discussion 3.1. Characterization of PbTiO3 Fig.1a presents the XRD patterns of PbTiO3 samples at the pH of 13.25 with
80
and 120
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different hydrothermal temperatures. As shown in the figure, the products obtained at are amorphous, and diffraction peaks of PbTiO3 could not be
detected. When the temperature was increased to 160
, a series of sharp and clear
diffraction peaks appear, which could be assigned to tetragonal phase PbTiO3 (JCPDS
was further increased to 220
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01-78-0298). No other impurities could be detected. As the hydrothermal temperature , the diffraction peaks intensity of sample was
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increased gradually, indicating that the crystallinities of samples improve. According to the Scherrer formula, the crystallite sizes of PbTiO3 samples obtained at 160 200
and 220
,
, calculated based on the data of (101) plane, are 53.2 nm, 67.6
nm and 78.7 nm, respectively.
Fig.1b shows the XRD patterns of PbTiO3 obtained at hydrothermal temperature
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200 °C with different pH values. It can be seen that the crystallinity of products is low at the pH value of 12.95. When the pH values are greater than 13, the diffraction peaks match well with tetragonal phase PbTiO3 (JCPDS 01-78-0298), and no peaks of
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any other phases or impurities were detected. The (001) peak intensity of samples obtained at the pH of 13.25 was increased suddenly, even stronger than (101) peak intensity, which may be due to anisotropic growth along the (001) facets [12].
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According to the Scherrer formula, the sample crystallite sizes calculated based on (101) plane are 51.3 nm, 67.6 nm and 67.7 nm when the pH value are 13.10, 13.25 and 13.40, respectively. It is obvious that the crystallite sizes of samples were increased with increasing pH values. Fig. 2 shows the TEM image and high-resolution TEM (HRTEM) image of the PbTiO3 prepared at the pH of 13.25 and the hydrothermal temperature of 200 °C. The as-prepared sample displays elliptical in shape, and its particle sizes could be estimated to be about 250 – 400 nm, which is much larger than its crystallite size (67.6 nm) obtained from the XRD data. The results indicate that there are multiple 4
ACCEPTED MANUSCRIPT domain in the particles of the sample and the PbTiO3 particles observed by TEM are mainly polycrystalline [13]. To illustrate the anisotropic growth along (001) plane of the as-prepared PbTiO3, the HRTEM image and the standard XRD pattern of PbTiO3 (JCPDS 01-78-0298)
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from powder diffraction file database are provided (Fig. 2b and Fig. 2c). In the standard pattern (Fig. 2c), the intensity of the (0 0 1) peak is much weaker than that of the (1 0 1) peak, which could be expressed as I (1 1 0) / I (1 0 2) = 0.25. In our case, the intensity of the (0 0 1) peak is even beyond that of (1 0 2) peak, the value of I (0 0
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1) / I (1 0 1) was increased to 1.59. The related HRTEM image of the as-prepared sample (Fig. 2b) displays the clearly resolved crystalline domain, an uniform
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interplanar spacing of 0.415 nm correspond well to the (001) plane of PbTiO3. Furthermore, the angle labeled in the corresponding fast-Fourier transform (FFT) pattern (inset in Fig. 2b) is 46.8◦, which is identical to the theoretical value obtains for the angle between the (0 0 1) and (1 0 1) planes. The results prove that the PbTiO3 sample anisotropically grew along the (0 0 1) plane.
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UV-Vis diffuse reflectance spectrum of the as-prepared PbTiO3 samples was analyzed to investigate the optical absorption performance. As shown in Fig. 3, the absorption edge of the PbTiO3 samples synthesized at the hydrothermal temperature
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of 200 °C and the pH of 13.25 extends to the visible light region, ranging from 200 nm to 450 nm. The steep shape of the spectrum indicates that the visible light absorption is ascribed not to the transition from the impurity level but to the band-gap
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transition [14]. Its band gap could be derived using the equation αhν = A(hν - Eg)n/2, where α, A, hν and Eg signify the absorption coefficient, proportionality constant, photon energy and band gap energy, respectively [15]. Considering PbTiO3 to be an indirect semiconductor (n = 4 is adopted), the plot of (αhν)1/2 versus hν affords the bandgap of PbTiO3 as shown in the inset of Fig. 3, and then evaluate the band gap Eg by extrapolating the tangent line to the hν axis intercept. The estimated band gap energy of the resulting samples is about 2.75 eV, which is consistent with previous reports [6,7]. The BET surface areas play a great role on the photocatalytical property of 5
ACCEPTED MANUSCRIPT samples. The specific surface areas of samples were investigated by using nitrogen adsorption and desorption isotherms. The surface areas of PbTiO3 prepared at the hydrothermal temperature of 200 °C are 3.33, 2.56 and 2.52 m2 g-1 at the pH values of 13.10, 13.25 and 13.40, respectively. The BET surface areas were decreased gradually
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with increasing pH value. However, the specific surface areas of samples prepared at pH values of 13.25 and 13.40 have little difference, which is consistent with the change of their crystallite sizes. The effects of the hydrothermal temperatures were also investigated, the surface area of PbTiO3 prepared at pH value of 13.25 are 2.06,
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2.56 and 1.74 m2 g-1 at 160, 200 and 220 °C, respectively. The specific surface areas of samples were firstly increased and then decreased. When the temperature was to 220
, the specific surface area were reduced, indicating
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increased from 200
that the crystalline degree was increased with increasing temperature, and accordingly specific surface area was decreased. 3.2. Photocatalytic performance
Fig. 4 shows the photocatalytic degradation of MO over PbTiO3 samples
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prepared at different conditions. It can be seen from Fig. 4a that the photolysis of methyl orange was very slow, and only about 3% was degraded after 180 min of illumination. The degradation of MO with as-prepared PbTiO3 samples in the dark
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condition gives similar results to the photolysis test, indicating that the adsorption of MO on the PbTiO3 samples was limited after the adsorption–desorption equilibrium was reached. However, about 91% of MO was decolored after 180 min of
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illumination over the PbTiO3 samples, indicating that the product exhibits relatively high photocatalytic activity. Although there might be photolysis of MO, the results of above comparison signify that MO degradation in the present study was through a photocatalytic process.
Fig. 4a presents the photocatalytic degradation of MO over PbTiO3 samples prepared at the pH of 13.25 with different temperatures. It is obvious that the hydrothermal temperatures had a great impact on photocatalytic properties of as-prepared samples. As the hydrothermal temperature was increased from 120 6
to
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, the photocatalytic activities of products were increased gradually. It could be
ascribed to the increase of the photocatalyst crystallinity degree with increasing temperature, which resulted in the reduction of the electron - hole recombination centers. However, when the hydrothermal temperature was increased to 220
, the
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photocatalytic activity of PbTiO3 was reduced inversely. It is due to the increase of particle sizes with increasing temperature, which led to the decrease of specific surface area and the active sites, so the photocatalytic activity of product was decreased.
prepared at 200
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Fig. 4b shows the photocatalytic degradation of MO over PbTiO3 samples with different pH values. When the pH value was increased from
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12.95 to 13.25, the photocatalytic activities of samples were increased, which can be attributed to the improvement of the catalyst crystallinity according to the XRD results. As the pH was further increased to 13.40, the photocatalytic activities of products were decreased, which may be related to the decrease of the (001) peak intensity, and it needs to be further studied. Generally, the optimal condition for the
value of 13.25.
and the reaction pH
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preparation of PbTiO3 is the hydrothermal temperature of 200
For comparison, the photocatalytic activity of PbTiO3 prepared by precipitation
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method was also tested. After 180 min of irradiation, the degradation efficiency of MO was only 31%, which is much less than that of PbTiO3 prepared by hydrothermal method under the same conditions.
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The stability and reusability of catalyst is an important indicator for its practical
application. To investigate its stability, the photodegradation cycle experiments of methyl orange over PbTiO3 samples were conducted, and the experimental results are shown in Fig. 5. The experimental conditions are accordance with the photodegradation experiment. It shows that the degradation efficiency of methyl orange over PbTiO3 is still up 73% after five photocatalytic cycles. The photocatalytic activity was not reduced considerably, indicating that the PbTiO3 sample has a high stability during the photodegradation process of methyl orange. It is very important in the application of the photocatalyst [16]. However, the photocatalytic activity of 7
ACCEPTED MANUSCRIPT PbTiO3 was decreased to a certain degree through the recycling. This may be ascribed to the presence of residual intermediates on the catalyst surface which reduced the surface activity centers, and then made the photocatalytic activity of the catalyst reduced [17]. Additionally, the XRD patterns of PbTiO3 samples after the cycle
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experiments was analyzed and shown in Fig. 6. It reveals that the phase and structure remained intact, implying that the as-prepared PbTiO3 has a high stability during photocatalytic oxidation of model pollutant.
The research of PbTiO3 as photocatalyst on the degradation of organic matter is
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scarce, and one of the reasons is that it may cause secondary pollution. So the dissolubility of PbTiO3 in the photocatalytic process was investigated by inductively
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coupled plasma atomic emission spectrometry (ICP-AES). The experimental process is similar to the photocatalytic process. The light illumination time was set to 15 h, and a sample was taken to analysis the amount of Pb2+ every 3h. After 1h dark adsorption, the concentration of Pb2+ dissolved in solution was reached to 0.139 mg·L−1, and the concentration of Pb2+ was increased to 0.212 mg·L−1 after 3 h of
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irradiation. But the concentration of Pb2+ was not increased much after 12 h of irradiation, the final cumulative concentration of Pb2+ was 0.226 mg·L−1. The total lead concentration dissolved in solution after 15 h of irradiation is lower than the limit
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of 0.5 mg·L−1, which meets the National Emission Standards of China (GB 25466-2010). Therefore, PbTiO3 does not cause secondary pollution when it is
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applied to wastewater treatment as a photocatalyst. 4. Conclusions
In summary, PbTiO3 photocatalyst was synthesized successfully by a facial
hydrothermal treatment. The hydrothermal reaction temperatures and the pH values of systems have great influence on the photocatalytic activities of PbTiO3, which were evaluated by the degradation of MO under simulated solar irradiation. According to the photodegradation efficiencies, the optimal condition is the hydrothermal reaction temperature of 200
and the pH value of 13.25. The PbTiO3 prepared at the
condition could degrade about 91% of MO after 180 min of illumination. In addition, the sample is highly stable during photocatalytic oxidation process of model pollutant, 8
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Acknowledgements This work was supported by National Natural Science Foundation of China (NO.
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21001096 and U1304204) and Foundation of He’nan Educational Committee
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(NO.13A150348).
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ACCEPTED MANUSCRIPT References [1] A. Yourdkhani, G. Caruntu, J. Phys. Chem. C 115 (2011) 14797-14805. [2] M. Yashima, K. Omoto, J. Chen, H. Kato, X. Xing, Chem. Mater. 23 (2011) 3135-3137.
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[3] P. Hu, J. Chen, J. Deng, X. Xing, J. Am. Chem. Soc. 132 (2010) 1925-1928. [4] M. Park, S. Hong, J. Kim, Y. Kim, S. Buhlmann, Y.K. Kim, K. No, Appl. Phys. Lett. 94 (2009) 092901-092903.
[5] H.G. Kim, D.W. Hwang, J.S. Lee, J. Am. Chem. Soc. 126 (2004) 8912-8913.
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[7] D. Arney, T. Watkins, P.A. Maggard, J. Am. Ceram. Soc. 94 (2011) 1483-1489. [8] V. Stankus, J. Dudonis, Mater. Sci. Eng. B 109 (2004) 178–182. [9] S. Yu, H. Huang, L. Zhou, Chem. Mater. 19 (2007) 2718-2720. [10] R. Velchuri, B. V. Kumar, V. R. Devi, G. Prasad, M. Vithal, Ceram. Int. 36 (2010) 1485-1489.
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[11] J. Moon, T. Li, C.A. Randall, J.H. Adair, J. Mater. Res. 12 (1997) 189-197. [12] Y. Li, J. Wang, H. Yao, L. Dang, Z. Li, J. Mol. Catal. A: Chem. 334 (2011) 116-122.
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[13] J.-L. Liu, L.-J. Zhu, Y. Pei, J.-H. Zhuang, H. Li, H.-X. Li, M.-H. Qiao, K.-N. Fan, Appl. Catal. A: Gen. 353 (2009) 282-287. [14] C. Zhang, Y. Zhu, Chem. Mater. 17 (2005) 3537-3545.
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[15] H. He, J. Yin, Y. Li, Y. Zhang, H. Qiu, J. Xu, T. Xu, C. Wang, Appl. Catal. B: Environ. 156–157 (2014) 35-43. [16] M. Shang, W. Wang, S. Sun, L. Zhou, L. Zhang, J. Phys. Chem. C 112 (2008) 10407-10411.
[17] L. Jing, B. Xin, F. Yuan, B. Wang, K. Shi, W. Cai, H. Fu, Appl. Catal. A: Gen. 275 (2004) 49-54.
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ACCEPTED MANUSCRIPT Figure captions: Fig. 1. XRD patterns of the PbTiO3 products prepared at different temperatures (a) and different pH values (b). Fig. 2. TEM image (a) and high-resolution TEM image (b) of the PbTiO3 prepared at
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pH = 13.25 and hydrothermal temperature 200 °C, the inset of (b) is the corresponding fast-Fourier transform (FFT) pattern. (c) XRD pattern of the PbTiO3 and standard XRD pattern of PbTiO3 (JCPDS no. 01-078-0298).
Fig. 3. The diffuse reflection spectrum of the PbTiO3 prepared at pH = 13.25 and
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hydrothermal temperature 200 °C.
Fig. 4. Photocatalytic degradation of MO over PbTiO3 samples prepared at pH =
different pH values (b).
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13.25 with different temperatures (a) and PbTiO3 samples prepared at 200
with
Fig. 5. Cycle degradation experiments of methyl orange over PbTiO3 samples.
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Fig. 6. XRD patterns of PbTiO3 samples after the photocatalytic cycle experiments.
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ACCEPTED MANUSCRIPT Research highlights:
Effects of pH and temperature on photocatalytic activity of PbTiO3 synthesized by hydrothermal method
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Yongyu Li1,2, Haijie Sun2, Ning Wang1, Wenxue Fang1, Zhongjun Li1*
◆PbTiO3 photocatalyst was synthesized by hydrothermal method.
PbTiO3 prepared by precipitation method.
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◆The photocatalytic activity of PbTiO3 is about 3 times higher than that of the
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◆The effects of the temperatures and the pH values of systems on the photocatalytic
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activities of PbTiO3 were investigated.